img: fix UAF on Reconfigure by refcounting

Before this commit, there was a use-after-free bug on Reconfigure:
- theme_finish() destroys lab_imgs for titlebar icons
- For some reason, undecorate() calls _create_buffer() in
  scaled-img-buffer.c, which calls img_render() on a destroyed lab_img.

So in this commit, the lifetime of lab_img is expanded to when the
scaled_img_buffers referencing it are all destroyed. This is achieved by
calling lab_img_copy() when setting a lab_img to scaled_img_buffer and
calling lab_img_destroy() when clearing a lab_img.

Now that scaled_img_buffer.img are always different, lab_img_equal() is
added to compare the content of scaled_img_buffer.img.

include/img/img.h

@@ -3,6 +3,7 @@
 #define LABWC_IMG_H
 
 #include <cairo.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <wayland-util.h>
 
@@ -72,4 +73,9 @@ struct lab_data_buffer *lab_img_render(struct lab_img *img,
  */
 void lab_img_destroy(struct lab_img *img);
 
+/**
+ * lab_img_equal() - Returns true if two images draw the same content
+ */
+bool lab_img_equal(struct lab_img *img_a, struct lab_img *img_b);
+
 #endif /* LABWC_IMG_H */